Recent years, as for metal resources such as rare metals, growing demand thereof and limited supply due to resource nationalism make market supply unstable, leading to supply anxiety and steep rise in metal prices. Such a situation leads to vigorous development in construction of technology and mechanism for recycling used products, and further energetic development in technology to reduce loss as much as possible in recovering and/or smelting process.
As methods for separating and recovering metals from metal-containing materials, there have been known an electrolytic smelting method, a flocculation precipitation method, a solvent extraction method, an adsorption method, and others. Among them, the adsorption method in which metals are recovered by using adsorbents is believed to be suitable for industrial practice, and various studies have been carried out.
Widely used adsorbents include one comprising a polymer substrate having an ion-exchange or chelating group introduced thereinto, the group having a high affinity for metals to be recovered. However, such an adsorbent has problems that the polymer substrate is generally less reactive and is limited in number and type of functional groups to be introduced.
In order to overcome the above problems, as a method for introducing ion-exchanging groups or chelating groups, an adsorbent has been known which comprising a porous polyolefin sheet having an adsorptive functional group-containing graft chain introduced thereinto by radiation-induced graft polymerization (Patent Document 1). Further, there has been known a metal ion adsorbent carrying extraction reagents, the adsorbent being obtained by graft-polymerizing polymerizable monomers carrying extracting reagents on a surface of a porous membrane (Patent Document 2).
As a method for introducing an adsorptive functional group by blending polymers, there have been known metal (particularly platinum metal) ion adsorbent particles obtained by extruding a composition comprising a polyethyleneimine and an ethylene-vinyl alcohol copolymer or a composition comprising a polyethyleneimine, an ethylene-vinyl alcohol copolymer and an amine-based polymer to form a strand, followed by pulverizing the strand (Patent Document 3).
Further, Patent Document 4 discloses an anion exchanging body having graft chains in which quaternary ammonium groups are introduced to an ethylene-vinyl alcohol copolymer by ionizing irradiation. In Examples of this document, an ethylene-vinyl alcohol copolymer is irradiated by ionizing radiation to introduce quaternary ammonium groups in a graft ratio of 60%.